Methods for producing silicon carbide, amorphous silica, or silicon metal from rice hulls or carbide of rice hulls are conventionally known.
In Patent Literature 1, a method for continuously producing silicon carbide and silicon nitride from carbide of rice hulls is described. In the method, the carbide of rice hulls is heated while being transported in a heating zone under presence of an inert gas nonreactive with carbon.
In Patent Literature 2, a method for producing a molded product of porous silicon carbide including composite structure of silicon carbide whiskers and silicon carbide fine particles made from carbide of rice hulls is described. In the method, the surface of pulverized carbide of rice hulls is treated and the carbide is molded after surface treatment. The molded product is then heat treated for removal of carbonized components.
In Patent Literature 3, a method for producing silicon metal made from silicicolous plants such as rice hulls is described. In the method, silica ash obtained by firing silicicolous plants is reacted with a metal such as aluminum by heating in an inert gas atmosphere.
In Patent Literature 4, a method for producing a silica raw material from rice hulls is described. In the method, rice hull powder is treated with pressurized hot water to adjust the ratio of silica components to organic components in the rice hull powder.
In Patent Literature 5, a method for producing high purity amorphous silica from rice hulls is described. In the method, rice hulls are treated with acetic acid-containing solution to separate alkaline components from the rice hulls. After separation of alkaline components, the rice hulls are washed and dried. The dried rice hulls are carbonized to form rice hull charcoal. The rice hull charcoal is fired to produce high-purity silica.
In Patent Literature 6, a method for producing active carbon is investigated focusing on carbon components of rice hulls. A hydrogen absorption method is described, including the steps of: carbonizing carbon material; subjecting the carbonized carbon material obtained in the previous step to alkali activation, accommodating the porous carbon made in the alkali activation step in a vessel, and introducing hydrogen into the interior of the vessel so as to have a pressure of 0.5 to 6 MPa in equilibrium state, while keeping the temperature in the interior of the vessel in the range of 77 to 150 K. The process has a problem that silica components are melted with an alkali salt and discarded.